This invention relates to the field of solar cells for converting solar energy into electrical energy. More particularly, this invention relates to dendritic web photovoltaic cell technology.
Solar cells comprising semiconductor devices employing the photovoltaic effect for converting solar energy into electrical energy have long been known. A known particular type of solar cell is made using the dendritic crystal web growing technique in which a source material, such as silicon, is melted in a furnace and slowly withdrawn using a seed crystal attached to a suitable mechanical drawing apparatus along a path from the source melt through a furnace exit port to a receiving station. Under suitable temperature controlled conditions and a proper withdrawal rate, the melt material forms a crystalline web having a dendritic cross-section which cools to a finished crystalline web. This process is more fully described in "Dendritic Web Silicon For Solar Cell Application", R. G. Seidenstecker, Journal of Crystal Growth, 39 (1977) PP. 17-22, the disclosure of which is hereby incorporated by reference.
In order to provide the photovoltaic effect, semiconductor junctions must be formed within the crystalline web using a suitable dopant technique. One popular technique employed is the dopant diffusion process in which a dopant material is diffused into the crystalline web via the two major web surfaces to provide first and second diffusion layers of opposite conductivity type. Diffusion has been done in the past using a gas diffusion process in which the dopant material is in the form of a gas stream which is permitted to flow past the web surfaces so that ions of the dopant materials can be diffused into the crystalline web material. This process must necessarily be carried out well downstream of the crystalline web formation site, and typically requires a separate diffusion station, which adds complexity to the dendritic solar cell production process. In addition, gaseous diffusion requires separate gas sources (for the dopant materials of opposite conductivity type) and carefully designed and fabricated gas flow paths to prevent the dopant gas streams from escaping to ambient or contaminating other equipment.